(a) Technical Field
The present invention relates to a method of manufacturing a membrane-electrode assembly (MEA) for a fuel cell. More particularly, the present invention relates to a method of manufacturing a 3-layer MEA in which a catalyst electrode layer is stacked on both surfaces of a membrane, and a method of manufacturing a 5-layer MEA in which gas diffusion layers (GDL) are stacked on both surfaces of a membrane with catalyst electrode layers disposed thereon.
(b) Background Art
In general, fuel cells are devices that convert the chemical energy of fuel directly into electrical energy by an electrochemical reaction of fuel and oxygen in air without combustion. Such fuel cells have attracted much attention as a zero emission power generation system and can be applied to the supply of electrical power for small-sized electrical/electronic devices, especially, portable devices, as well as to the supply of electrical power for industry, home, and vehicle.
A polymer electrolyte membrane fuel cell (PEMFC) has advantages of higher output density, faster response, and simpler system configuration, and thus extensive research for using the PEMFC as a vehicle power source or a stationary power generation system has continued to progress.
The PEMFC includes a membrane-electrode assembly (MEA), which is a major component and positioned at the most inner portion. The MEA includes an anode, a cathode and an electrolyte membrane disposed therebetween.
Referring to FIG. 1, catalyst electrode layers 3, i.e., the anode and the cathode are formed by uniformly coating a desired amount of catalyst on both surfaces of a polymer electrolyte membrane 4. Gas diffusion layers (GDLs) 2 are positioned at the outside of the MEA, i.e., on the surfaces where the catalyst is present, and separators 1 having flow fields for supplying fuel and exhausting water produced by the reaction are positioned at the outside of the GDL 2.
In general, a unit cell of the PEMFC includes one polymer electrolyte membrane, two catalyst electrode layers, two GDLs, and two separators. A plurality of such unit cells are stacked to form a fuel cell stack of a desired scale.
In the MEA, an oxidation reaction of hydrogen occurs at the anode of the fuel cell to produce hydrogen ions and electrons. The thus produced hydrogen ions and electrons are transferred to the cathode through the polymer electrolyte membrane and a conducting wire, respectively.
At the same time, a reduction reaction of oxygen occurs at the cathode receiving the hydrogen ions and electrons from the anode to produce water. Here, electrical energy is generated by the flow of the electrons through the conducting wire and by the flow of the protons through the polymer electrolyte membrane.
The MEAs are generally classified into a 3-layer MEA and a 5-layer MEA. The 3-layer MEA includes an electrolyte membrane and catalyst electrode layers disposed thereon. The 5-layer MEA includes an electrolyte membrane, catalyst electrode layers and gas diffusion layers. Conventionally, the 5-layer MEA is formed by bonding gas diffusion electrodes (GDEs) to both sides of the electrolyte membrane using a hot pressing process.
It is known that the performance of 3-layer MEA is better than that of the 5-layer MEA under the same conditions. The reason for this is that the interfacial resistance between the electrolyte membrane and the catalyst electrode layer of the 3-layer MEA is lower than that of the 5-layer MEA.
The interfacial resistance between the electrolyte membrane and the catalyst electrode layer has a direct effect on the MEA performance, and the MEA performance has a direct effect on the fuel cell performance. Accordingly, it is understood that, if the interfacial resistance of the MEA is reduced, it is possible to improve the fuel cell performance remarkably.
A typical method of manufacturing a 3-layer MEA is catalyst-coated membrane (CCM) method in which catalyst is coated on an electrolyte membrane. A typical method of manufacturing a 5-layer MEA is catalyst-coated GDL (CCG) method in which catalyst is coated on GDLs to form an anode and a cathode and the catalyst-coated GDL is bonded to a polymer electrolyte membrane.
In more detail, the 3-layer MEA is manufactured in such a manner that a catalyst slurry of low concentration is directly spray-coated on the electrolyte membrane using a spray gun to form a catalyst electrode layer, or the catalyst slurry is coated on a film and then subjected to a decal method. In the decal method, a polymer film as a release film coated with the catalyst slurry is hot-pressed onto a membrane such that an electrode is transferred onto the membrane.
The 3-layer MEA manufactured by directly spray-coating the catalyst on the membrane has an advantage that it is possible to minimize the interfacial resistance between the catalyst electrode layer and the membrane; however, since the catalyst slurry of low concentration should be spray-coated repeatedly on the membrane, it takes a long processing time and there is inevitably a loss of the expensive catalyst in terms of the spray process characteristics.
Moreover, it is not easy to directly spray-coat the catalyst on the membrane, and the coating method is limited only to the spray coating method. The reason for this is that, if the membrane is in direct contact with a solvent contained in the catalyst slurry, the membrane is swollen and deformed. Thus, it is difficult to directly spray-coat the catalyst on the membrane by other methods.
In the event that the catalyst is directly spray-coated on the surface of the membrane using the spray coating method, the solvent should be removed repeatedly after the direct spray-coating in order to reduce the amount of solvent coming in contact with the membrane, prevent the catalyst electrode layer from being cracked, and minimize the deformation of the membrane.
In the event that other methods are employed, since a large amount of solvent present in the catalyst slurry is in direct contact with the membrane, the membrane is deformed, the catalyst electrode layer is not formed uniformly, and thus it is impossible to manufacture the MEA.
For such reasons, the following method is generally used. That is, the catalyst is coated on a release film, in which no deformation is caused by the solvent, using a spray, a screen printing, or a casting knife and dried at a high temperature. Then, the resulting release film is placed on the membrane and pressed at a high temperature and pressure, thus transferring the catalyst electrode layer coated on the release film to the membrane.
Such a method is called a decal method. The decal method has an advantage in that the release film may not be deformed by the solvent since the catalyst electrode layer is coated on the release film. However, since it employs the release film, the manufacturing cost is increased and it additionally requires the pressing process, compared with the method of direct spray-coating the catalyst on the membrane.
Moreover, the decal method has another disadvantage in that, since the catalyst electrode layer in a solid phase, from which the solvent is removed, is transferred to the membrane, the contact area between the catalyst electrode layer and the membrane is reduced, and the interfacial resistance is increased, compared with the direct coating method.
FIG. 2 is a schematic diagram illustrating a conventional process of forming a catalyst slurry on a release film. As shown in the figure, a catalyst slurry 13 is coated on a polymer film 11 as a release film using a casting knife in the shape of a bar or an applicator 12. Since the above components have a flat film surface and a micro-scale gap, the catalyst slurry 13 is passed through the gaps and coated by the thickness of the gap, thus forming a catalyst electrode layer 13′ having a small thickness on the polymer film 11.
Such a method of coating the catalyst slurry on the film surface is called a casting or bar coating method.
FIG. 3 is a schematic diagram illustrating a conventional hot pressing process, in which a release film 11 including a catalyst electrode layer 13′ is compressed on both surfaces of a membrane 14, respectively, by applying heat to transfer the catalyst electrode layers 13′ on the membrane 14, and FIG. 4 is a diagram illustrating a configuration of a 3-layer MEA manufactured by the decal method.
According to the conventional MEA manufacturing methods using the bar coating and decal methods of FIGS. 2 to 4, the catalyst slurry is placed on the expensive polymer film 11 such as polyethyleneimine (PEI), silicon coated polyethyleneterephthalate (PET), and polytetrafluoroethylene (PTFE) coating film and, then, the applicator 12 is moved in the casting direction to coat the catalyst slurry 13 on the polymer film 11 in a constant thickness.
At this time, the thickness of the coated catalyst slurry 13 can be adjusted by the applicator 12.
After the catalyst slurry is coated, the resulting polymer film 11 is dried in an oven at 60° C. to 100° C. for 30 minutes to completely remove the solvent contained in the catalyst slurry 13. The dried polymer film 11 is placed on both surfaces of the membrane 14 and subjected to the hot pressing process at 100° C. to 150° C. for 1 to 5 minutes, thus forming a 3-layer MEA (from which the polymer film is to be removed).
In the thus formed 3-layer MEA of FIG. 4, since the catalyst electrode layer 13′ in a solid phase, from which the solvent is completely removed, is transferred to the membrane 14, the interfacial adhesion between the catalyst electrode layer 13′ and the membrane 14 is reduced compared with the direct coating method, and the manufacturing cost is increased due to the use of expensive release film.
Meanwhile, the 5-layer MEA is manufactured in such a manner that an electrolyte membrane is disposed between two gas diffusion electrodes (GDEs) and pressed at a high temperature and high pressure, in which each of the GDEs is prepared by directly coating the catalyst on a GDL.
The thus formed 5-layer MEA has an advantage of a simpler manufacturing process; however, since the catalyst electrode layer and the membrane in the same solid phase are bonded to each other by the hot pressing process, the contact area between the catalyst electrode layer and the membrane is reduced and thus the interfacial resistance is increased more than that of the 3-layer MEA.
Moreover, it has a further drawback in that the method of coating the catalyst slurry on the GDL is limited to the spray coating method in which the loss of catalyst is considerable.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.